cg or trim issue?

[danger-russ]

I just madined my new plane. It's a basic trainer style plane with a 45 in span,6.5 in chord, clark y airfoil, 6* dihedreal ,8.6 oz sq ft wing loading,2.5*+ wing incedence 2* down and right on the motor,cg at 25% MAC.

I got her trimed strait and level at cruise speed about 20 mph.She flew great no problems at all.She got stuck in a thermal and I had a heck of a time getting her out.on landing near her stall stall speed with the throttle cut way back she wanted to nose up about 20* with the sticks neutral.
I'm thinking she might be a little tail heavy,but the down trim in the motor cancles it out under power.

I wondering wheather I should just add a little more down trim on the elevator or move the cg forward a little?

[Brandano]

If you have comfort space to move the CG forward it shouldn't hurt. Worst that can happen is that you will lose some elevator authority. If it's at all possible I'd increase the stab size.

[kcaldwel]

If the elevator sensitivity was OK (not too touchy) the CG is likely OK. CG location sets the stability margin, and therefore the elevator sensitivity. Other trim issues should be set with motor thrust and elevator trim/incidence adjustments. If the elevator is too sensitve, then you would move the CG forward.

You may have a bit too much down thrust, as you noted, and that results in carrying too much up elevator under power. This would result in the pitch-up when the power is cut. If you take out some down thrust, you can reduce the up trim for powered flight. You can adjust this until you don't have a pitch trim change with throttle changes.

Kevin

[gunsguns]

Hi,
2,5° incidence might be too much for this design.

You can either move the C.G forward - gives you a greater stability margin.

Or reduce the incidence stepwise, to 2° degrees and probably further.

In both cases it will be necessary to reduce the downthrust of the engine.

regards

Andi

[danger-russ]

If the elevator sensitivity was OK (not too touchy) the CG is likely OK. CG location sets the stability margin, and therefore the elevator sensitivity. Other trim issues should be set with motor thrust and elevator trim/incidence adjustments. If the elevator is too sensitve, then you would move the CG forward.

You may have a bit too much down thrust, as you noted, and that results in carrying too much up elevator under power. This would result in the pitch-up when the power is cut. If you take out some down thrust, you can reduce the up trim for powered flight. You can adjust this until you don't have a pitch trim change with throttle changes.

Kevin

This is whats confusing me the elevator sensitivity is perfect,and It trimed out with 2 clicks down elevator.

[kcaldwel]

2.5 degrees of wing incidence to the chord line on a cambered Clark Y type airfoil is more like 3.5 degrees to the zero lift line. With another 2 degrees of down thrust from, there you effectively have about 5.5 degrees of down thrust, which is too much.

I think you'll find if you take it up to altitude and throttle back, you will have to add a bunch of down trim to make it glide decently. You can either add down trim permanently, or reduce the wing incidence (or increase the stab incidence, but that won't also be reducing the effective down thrust). Once that is done, you will likely still find you need up trim under power. If so, you should reduce the down thrust until there isn't a big trim change between power on and off.

Kevin

[danger-russ]

The more I think about it,it might be piliot error.This is my first plane after a year of only flying heli's.I might be inputting up elevator trying to land like heli and not realizing it.I'll have to fly it some more before I start making changes.

[JetPlaneFlyer]

Russ,
I think that you may be on the right lines there. If the model flies fine under all conditions except for final landing approach then in all likelihood you are simply trying to slow the model up too much by hauling back on the stick.
If the CG, rigging angles or thrust line were badly out of whack I’d be confident that you would notice it elsewhere in the flight envelope.

Steve

[danger-russ]

It was a trim issue I got two flights in this morning.I had to add about 6 more clicks of down on the elevator and all is good.It's kinda nice to fly for a full hour
on a 1500 3s pack.